Detailed explanation of method examples on how to use javascript and jquery to detect the existence of dom nodes

1. JavaScript determines whether a DOM node exists in the page

The JavaScript native function does not provide a method to determine whether a DOM node exists. We usually obtain DOM nodes through the document.getElement.. method, which will return an object. Array collection, we can access each object of this collection through object[0], object[1]. Since an array set is returned, there is a length attribute, and length greater than or equal to 1 means that the DOM node exists in the page.

Code:

# #

Object.prototype.exist = function(){ 
 if(typeof this !='undefined' && this.length>=1){
  return true;
 }
 
 return false;
};

Use:

Assume that the page has the following nodes

##

<p>这里是p节点</p>
<p>这里是p节点</p>
<span>这里是span节点</span>

Judge whether the node is on the page:

var is_exist = document.getElementsByTagName('p').exist();
alert(is_exist); // true
 
var is_exist = document.getElementsByTagName('span').exist();
alert(is_exist); // true
 
var is_exist = document.getElementsByTagName('p').exist();
alert(is_exist); // false

Note: If you use the document.getElementById() method to obtain the object, you cannot use the exist() method, because the method of obtaining the node object based on the ID is If the node cannot be obtained, an empty object will be returned and the prototype exist() function will not be integrated, so an error will be reported! So if you are fetching the object based on the ID, you can directly if(obj). This way you can determine whether the DOM node exists in the page.2. jQuery determines whether the DOM node exists in the page.

Add prototype:

(function($) {
 $.fn.exist = function(){ 
  if($(this).length>=1){
   return true;
  }
  return false;
 };
})(jQuery);

Usage method:

If the page has the following DOM nodes

这里是id=a节点
<p>这里是p节点</p>
<p>这里是p节点</p>
<span>这里是span节点</span>

Judgment:

alert($('#aaa').exist()); // false
alert($('#a').exist()); // true
alert($('p').exist()); // true
alert($('p').exist()); // false

The above two methods actually determine whether the object exists based on the length attribute of the object collection.

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